Craniosynostosis is a common human disorder involving premature fusion of cranial sutures. Syndromic forms of craniosynostosis, such as Crouzon syndrome, are associated with pronounced midfacial hypoplasia and often result in severe malocclusion and respiratory and feeding problems. Although rare, this disorder has serious lifetime functional, esthetic, and social consequences and is often devastating to parents and children alike (NIDCR Strategic Plan). Several genes have been implicated in producing syndromic craniosynostosis, including Fibroblast growth factor receptor 2 (FGFR2). However, although a genetic basis of this serious condition has been established, the mechanisms that connect genotype to phenotype remain unknown. Fgfr2 mutations in mice have been shown to increase proliferation at cranial vault sutures but reduce proliferation at cranial base synchondroses in the same individuals, suggesting that Fgfr2 may act on the osteoblastic and chondrogenic lineages in different ways. Recent work has focused primarily on the vault sutures and the osteoblastic lineage. The role of Fgfr2 in the cranial base, which ossifies endochondrally, is less well understood, and analysis is usually restricted to midline sections through the spheno-occipital and inter- sphenoidal synchondroses. Preliminary data suggest that the first evidence of bony bridging in a gain-of- function Fgfr2 mutant occurs between the presphenoid and vomer, a junction between endochondral and intramembranous ossification sites. The combined defects of osteogenic and chondrogenic lineages may explain the extreme dysmorphology observed at the spheno-septal junction, and yet the mechanisms of Fgfr2 action in this region have never been directly addressed. The purpose of this study is to test the effect of a Fgfr2 gain-of-function mutation on suture biology in the sphenoethmoidal complex. The proposed research uses the Fgfr2+/C342Y mouse as a model for Crouzon syndrome. Stem cell proliferation, differentiation and apoptosis will be evaluated at the articulations of presphenoid with cartilage of the mesethmoid and with intramembranous ossification centers of the vomer in a developmental series of Fgfr2+/C342Y mice and wild-type littermates. Comparative analyses will be performed at the coronal sutures and spheno-occipital and inter-sphenoidal synchondroses. Multiple time points are analyzed because of evidence suggesting that Fgfr2 may play different roles depending on maturation of cells (proliferating vs. differentiating). In addition, this study will demonstrate how cellular abnormalities affect the onset and progression of craniofacial dysmorphology by integrating histological and shape data on the same animals. Ultimately, we will improve the ability to predict surgical outcome in syndromic craniosynostosis and implement pharmacological interventions to prevent suture fusion prior to the onset of dysmorphology. PUBLIC HEALTH RELEVANCE: The syndromic forms of craniosynostosis are a significant public health concern, because affected children require highly-invasive neurocranial surgery and repeated reconstructive procedures throughout childhood, with associated risks and expenses. The development of better surgical and pharmacological interventions requires that we understand how single gene mutations (for example, the mutation in FGFR2 that causes Crouzon syndrome) produce profound problems in the development of the skull. This study tests the cellular mechanisms that are affected by the Fgfr2+/C342Y mutation at the spheno-septal junction in a mouse model for Crouzon syndrome and demonstrates how sutural malformation affects cranial dysmorphology.